Hot water overflow - Sealed boiler system?


Help. Boiler is making a funny noise and hot water seems to be leaking out of some sort of overflow.
Its a sealed system I think - one big boiler and two small bits.
Also, any hot water is not kept hot for long. Is it leaking away?
Is the pressure too high or something? I can see a clock display but thats only just under 1 bar.
Any ideas?
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Do you have an unvented cylinder?
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I think its unvented.
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